1. Field of the Invention
Apparatuses consistent with the present invention relate to a small ultra wideband (UWB) antenna, and more particularly to a small UWB antenna designed to have a unidirectional radiation pattern by combining a loop radiator and a dipole radiator.
2. Description of the Related Art
All antennas are used to convert an electric signal into a specified electromagnetic wave to radiate the converted electromagnetic wave to free space, or to convert a received electromagnetic wave into an electric signal. UWB technology means a wireless transmission technology that directly transmits and receives an impulse signal without using an RF carrier. A UWB antenna is an antenna that can transmit and receive an impulse signal using a frequency band in the range of 3.1 to 10.6 GHz.
This UWB technology refers to a communication method that can achieve a high-speed data transmission using an ultra low power as it uses a very wide frequency band, unlike the existing narrow-band communication method. Accordingly, it can be applied to portable communication appliances that have been rapidly developed.
An antenna having been used in currently developed portable communication devices is required to satisfy the following conditions: being capable of performing UWB signal transmission/reception, having unidirectional radiation pattern, and being subminiature. The radiation pattern means the shape of an effective region where an antenna can radiate or sense electromagnetic waves. Since communication is possible in the case where the radiation pattern is formed in the direction of a base station, a portable communication appliance requires a unidirectional radiation pattern.
FIG. 1 is a view illustrating the structure of a Vivaldi antenna known in the art. Referring to FIG. 1, the antenna includes a power feeding part 11, an excitation part 12, a slot 13, a dipole radiator 14, and a substrate 15 that supports the above-mentioned components. The structure of such a Vivaldi antenna is disclosed in U.S. Pat. No. 5,428,364. When an external electromagnetic energy is supplied through the power feeding part 11, the excitation part 12 is excited. Accordingly, the electromagnetic energy transmitted along the power feeding part 11 is transferred to the slot 13 the width of which is gradually widened. The transferred electromagnetic energy is converted into an electromagnetic wave in the air at a right end part of the slot 13, and the electromagnetic wave is radiated in one direction as indicated by an arrow in FIG. 1.
This Vivaldi antenna can perform UWB signal transmission/reception and has a unidirectional radiation pattern. However, it requires an impedance matching in order to secure the radiation characteristic of the desired whole frequency band and to transmit electromagnetic energy provided from an external source without loss. In order to achieve the impedance matching, the size of the antenna should be increased as the wavelength of the wave is lengthened.
Consequently, in order to perform a low frequency band communication, the size of the antenna should be increased, and this causes a difficulty in miniaturization of the communication appliance.
FIG. 2 is a view illustrating the structure of a substrate type dipole antenna. Referring to FIG. 2, the substrate type dipole antenna includes a substrate 21, a first radiator 22, second radiators 23a and 23b, a feeder 24, and a signal supply part 25. The antenna structure of FIG. 2 is disclosed in U.S. Pat. No. 6,642,903, the detailed explanation thereof will be omitted.
In the substrate type dipole antenna of FIG. 2, the first radiator 22 and the second radiators 23a and 23b, which are prepared as wide plane conductors, are laminated on the substrate 21 to implement a wideband antenna. The electromagnetic energy supplied from the signal supply part 25 is applied to the feeder 24. The feeder 24 and separations 26a and 26b formed on the right and left of the feeder 24 constitute a feed region 30. The fed electromagnetic energy is converted into electromagnetic waves by the first radiator 22 and the second radiators 23a and 23b, and the converted electromagnetic waves are radiated in the direction of an arrow. This substrate type dipole antenna has the advantage in that it can transmit a UWB signal and can be fabricated with a relatively small size, but has the problem that it cannot have a unidirectional radiation pattern.
In addition to the Vivaldi antenna and the substrate type dipole antenna as described above, “Microstrip Patch Antenna,” by Weigand et al, IEEE Trans. Antennas Propagat. vol. 51, no. 3, March 2003, is known. Although this microstrip patch antenna has unidirectional radiation pattern and can be subminiaturized, it has the problem that it has a narrow bandwidth.